Most types of electronic equipment, e.g., TV sets, aircraft navigation devices and the like, incorporate circuit boards made of a thin, flat dielectric sheet. The sheet has resistors, capacitors, integrated circuits and other components mounted on it. While some boards are "hard-wired" using conventional soldered wiring, printed circuit boards (or "PC boards") are more common. Such PC boards employ flat foil strips (applied by a process akin to printing) as the "wiring" for component interconnection.
Many applications for circuit boards involve hostile operating environments particularly including temperature extremes and, sometimes, rapid excursions between such extremes. Unless recognized in board construction and testing, such environments can cause premature failure of the board per se and/or of the components mounted thereon. For example, radios used in mobile equipment such as military vehicles often experience extremes of heat and cold. And aircraft electronic gear similarly experiences such extremes--but over a much shorter time span. For example, a military aircraft may be at summer desert temperatures and at subzero temperature at high altitude only a few minutes later.
To help assure that circuit boards provide the requisite degree of reliability in such applications, board manufacturers often subject them to temperature tests by placing them within environmental chambers capable of producing rapid and extreme changes in temperature. For example, such a chamber might provide a temperature change of from -40.degree. F. to over 200.degree. F. in about 30 minutes. A prominent designer and manufacturer of such environmental test chambers is Thermotron Industries, Inc., of Holland, Mich.
A significant concern of circuit board manufacturers is the ability to test large quantities of boards economically. Testing of several hundred circuit boards per working shift is not unusual. Where such boards fill a high production requirement such as for passenger automobiles, circuit board test facilities are absolutely required to keep pace with the demands of modern, rapid production. Clearly, the environmental test chamber and its companion chamber-inserted carrier must avoid "bottle-necking" the production process. Until the invention, efforts to avoid such delay have not been uniformly successful.
The ability to quickly test large quantities of circuit boards involves at least two primary factors, namely, (a) the size, in BTU capacity, of chamber heating and refrigeration units and (b) the ability to quickly load the carrier with boards to be tested and unload such boards after testing is complete.
The ability to achieve the latter is significantly impaired if the same carrier is used to test circuit boards of differing sizes (e.g., length and/or height) or if different mother boards are required to interface the circuit boards with the test instruments. The invention is related to carrier configuration and uniquely resolves problems attending test of circuit boards including boards of disparate types.
One type of known carrier system is simply a cart on which the test chamber user places pallets containing products to be tested. The pallet-bearing cart is rolled into the chamber by a transport vehicle resembling a small lift truck. After the cart is "spotted" in the chamber, the vehicle is withdrawn and the chamber doors closed. In a variation of this arrangement, the cart incorporates an integral door so that when the cart is properly positioned, the door is automatically placed for latching closed. Such arrangements have significant "mass" embodied in the amount of steel and other materials used in its construction. Unnecessary mass acts as a heat absorber and can require that a test chamber have larger heating and refrigeration units to provide specified temperature excursions within a limited time.
Another type of "carrier" involves multiple drawers individually inserted into racks. Each drawer is loaded with circuit boards hooked to one or more common connectors by "pigtail" cables. Drawers are connected to external instrumentation by multi-plug back plane connectors. An arrangement of this type is shown in Environmental Engineering magazine, issue of June, 1991.
Other types of carriers are shown in U.S. Pat. Nos. 4,683,424 (Cutright et al.) and 4,986,778 (Liken et al.). The carrier shown in the Cutright et al. patent uses a frame in which multiple pull-out drawers are mounted. The arrangement somewhat resembles a multi-drawer file cabinet. Drawers are withdrawn, loaded with individual circuit boards plugged to a mother board and thereupon re-inserted in the frame. When the drawer is fully inserted, an electrical plug at the rear of the drawer connects with a stationary back plane plug at the rear of the drawer compartment.
The carrier shown in the Liken et al. patent involves a relatively thin frame-like structure having four openings into which may be fitted individual circuit boards for test. Structures are made with different openings and circuit board mounting brackets to accommodate boards of different sizes. In turn, the frames are inserted in parallel, spaced relationship into a larger carrier-like rack.
Generally stated, known carriers have a fixed number of non-movable shelves, mother boards mounted in a predetermined part of the carrier and are made of pieces of stainless steel tubing welded together. Such carriers are configured to test a predetermined number of circuit boards of a specific size or sizes.
Notwithstanding the above-described carrier arrangements and their general acceptability to users of environmental chambers, there is a significant need for a carrier which can be readily adapted to a variety of circuit board types and sizes. A carrier having such a capability and having a low mass for reduced thermal loading of heating and refrigeration units would be an important advance in the art.